Effects of visco-thermal losses in metamaterials slabs based on rigid building units

Potential applications of negative-index acoustic metamaterials are strongly limited by absorptive effects of different origin. In this context, we present an investigation of the visco-thermal effects on the acoustic properties of double-negative metamaterials based on specifically designed rigid units with subwavelength dimensions. It is shown that visco-thermal losses dissipate about 70% of the acoustic energy associated to the excitation of monopolar and dipolar resonances, leading to the suppression of negative refractive index. Our numerical simulations based on the Boundary Element Method (BEM) are in excellent agreement with recent experimental data showing the quenching of the double-negative transmission peak. The BEM numerical model, which has been specifically adapted to this purpose, has also been validated against an equivalent Finite Element Method model. We also present the results and discuss the differences of visco-thermal effects on monopolar resonances leading to negative bulk modulus metamaterials, and Fabry-Perot resonances in metamaterial slabs.